A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In certain classes of lithographic apparatus, especially in lithographic apparatus operative in the EUV (extreme ultraviolet) area of the electromagnetic spectrum, a desire exists to carry out the lithographic process in vacuum conditions to optimize and/or enable process conditions for the lithographic process. To this end, large parts of the apparatus are contained in a vacuum housing that is subject to vacuum pressure. In this housing, the lithographic process is carried out, specifically, the substrate is irradiated in a vacuum environment. While a substantial amount of equipment is often needed to carry out this lithographic process, specifically, at least one of a support constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, in a vacuum environment, a desire exists to keep the parts exposed to vacuum pressure levels to a minimum. Indeed, the larger the exposed vacuum surface area is, the larger is risk of entrapping contaminants, such as water particles or hydrocarbons. In particular, water has a tendency to stick to surfaces and is often difficult to evacuate. The presence of such contaminants may cause serious shortening of performance and service lifetime of machine parts, and optical elements in particular.
However, the nature of these apparatus is such that it is difficult to reduce the effective area of surfaces exposed to the vacuum environment because of numerous other constraints. For example, because of image resolution requirements, a variety of telemetry and sensor elements are present that cannot easily be reduced or prepared for vacuum conditions. Also, in vacuum lithographic environments, various parts of the machine may be subject to different vacuum regimes, where it is also possible that a controlled leakage of service gases, for example, for creating a optimal lithographic process conditions, be present.
All these factors may contribute to serious challenges for bringing the lithographic apparatus into a vacuum operating condition. One of the main challenges is to bring the down-time of such machines to a minimum, in order to reduce costs. A conventional approach to achieve this is to heat the vacuum housing of the machine in which the lithographic apparatus is housed, in particular, the substrate table and/or the projection system. Furthermore, in order to minimize vacuum exposed areas, the tendency is to position heating elements outside the housing and to “bake-out” the vacuum exposed parts in the preparation process of bringing the lithographic apparatus into vacuum operating condition.